The bis(fluorosulfonyl)imide anion (FSI−) is useful in various applications including electrolytes in electrochemical devices such as batteries and capacitors. The present invention is directed at the synthesis of hydrogen bis(fluorosulfonyl)imide (HFSI), which is a useful intermediate for salts and ionic liquids containing this anion.
Hydrogen bis(fluorosulfonyl)imide, its corresponding salts and ionic liquids comprising the FSI anion have been shown to be useful in a wide variety of applications including, but not limited to, as electrolytes in lithium ion batteries and ultracapacitors. Hydrogen bis(fluorosulfonyl)imide is a relatively strong acid and forms various stable metal salts. The lithium salt of bis(fluorosulfonyl)imide (i.e., LiFSI) has shown to be particularly useful in batteries and ultracapacitors.
Despite the advantages of compounds containing the FSI anion, the corresponding salts and ionic liquids thereof, no large scale commercial production exists. While many processes for producing HFSI are known, each of the known methods for synthesizing HFSI has disadvantages. For example, one method for synthesizing HFSI uses urea (NH2CONH2) and fluorosulfonic acid (FSO3H). One of the major disadvantages for this process is the toxicity and corrosiveness of fluorosulfonic acid. Moreover, it is difficult to control this reaction due to local overheating during the addition of fluorosulfonic acid to the reaction mixture. This difficulty in controlling the reaction results in an unpredictable yield of the desired product.
Another method for synthesizing HFSI involves fluorinating bis(chlorosulfonyl)-imide (i.e., HCSI) with arsenic trifluoride (AsF3). In this reaction, HCSI is treated with AsF3. Arsenic trifluoride is toxic and because it has a high vapor pressure, it is particularly difficult to handle on an industrial scale. A typical reaction uses 1:8.6 ratio of HCSI to AsF3. This means a large excess of highly dangerous arsenic trifluoride is used.
HFSI can also be prepared by the fluorination of HCSI with antimony trifluoride (SbF3). The antimony trichloride byproduct of this reaction has both high solubility in HFSI and a very similar boiling point, making it very difficult to separate from the desired product. The product of this reaction is also typically contaminated with chloride, which renders the product unsuitable for electrochemical applications.
One of the simplest reactions for producing HFSI is to react hydrogen bis(halosulfonyl)imide (HXSI) with hydrogen fluoride, where X is a nonfluoro-halide. However, this reaction has been reported to produce a relatively poor yield and involved reacting HCSI with excess anhydrous HF at high temperature. See, for example, U.S. Pat. No. 7,919,629. In particular, Example 10 of U.S. Pat. No. 7,919,629 discloses reacting HCSI with anhydrous HF at various temperatures. The best yield was 55% at 130° C. for 2 hours. Some reaction was observed after 12 hours at 30 and 50° C. (<10% yield). The inventor of this patent observed fluorosulfonic acid as a degradation product. More significantly, the inventor concludes that “ . . . the synthesis of HFSI with HF is not satisfactory.” (Emphasis added).
Accordingly, there is a need for a relatively safe and/or simple method for producing a high yield of hydrogen bis(fluorosulfonyl)imide.